A seat heater comprises a heating element, which is typically a low-resistance conductor (in the form of a wire, cable, conductive trace printed on an insulating substrate, or the like) for being arranged under the seat cover. In operation, a direct current of several amperes is sent through the heating element to generate heat.
Seat heaters may be standard or supplementary equipment on automotive vehicles. A vehicle seat may furthermore be equipped with a capacitive sensor to detect the occupancy state thereof. The detected occupancy state may then be used as an input for various vehicular applications, such as, e.g., a seat belt reminder, occupant classification for low-risk airbag deployment, or the like. The seat heater, in particular the heating element, can affect operation of such seat-based capacitive occupancy sensors when these devices are located in the same area of the seat. A reason for this is that the antenna electrode of the capacitive sensor and the heating element, which appears to be ground to the capacitive sensor, may form a substantially larger capacitance than the capacitance to be measured.
This problem has been addressed e.g. by US 2009/0295199, which discloses a combined seat heater and capacitive sensor. The heating element is coupled both to a heating circuit for being supplied with electrical current for generating heat and to an occupant sensing circuit for sensing the presence of an occupant near the heating element. The arrangement operates by periodically disconnecting the heating circuit from the heating element and connecting the occupant sensing circuit. Because of the sensitivity of the measurements required by the occupant sensing circuit, it is necessary to electrically isolate the heating current source from the heating element to prevent interference with the occupant sensing circuit. Nevertheless, if the heating control circuit has open-switch impedance that combines with and influences the electric field impedance, the accuracy and reliability of occupant detection is reduced. US 2009/0295199 thus proposes an isolation circuit interposed between the heating element and the heating circuit. Specifically, each of the two terminals of the heating element is connected to the heating power supply via two transistors disposed in series. When the arrangement is in capacitive sensing mode, the nodes between each pair of transistors are actively kept at the same potential as the heating element by means of respective voltage followers in order to neutralize any open-switch impedance of the transistors.
The very same idea has already been disclosed in U.S. Pat. No. 6,703,845. As an alternative to transistors, that document also proposed inductors to achieve high impedance at the frequency of the oscillating signal (used for capacitive sensing) between the heating element and the power source of the heating circuit. As in US 2009/0295199, a voltage follower maintains the intermediate nodes substantially at the same potential as the heating element in order to effectively isolate, at the frequency of the oscillating signal, the power supply of the heating circuit from the heating element.
The devices disclosed in U.S. Pat. No. 6,703,845 and US 2009/0295199 have in common that they isolate the heating element from its power supply at least for the frequencies of the capacitive measurement.